Plasma Renin Activity

Renin secretion from the kidneys is the first stage of the renin-angiotensin-aldosterone cycle, which controls the body's sodium-potassium balance, fluid volume, and blood pressure. Renin is released into the renal veins in response to sodium depletion and blood loss. This test is a screening procedure for renovascular hypertension but does not unequivocally confirm it.

Purpose

• To screen for renal origin of hypertension
• To help plan treatment of essential hypertension, a genetic disease often aggravated by excess sodium intake
• To help identify hypertension linked to unilateral (sometimes bilateral) renovascular disease by renal vein catheterization
• To help identify primary aldosteronism (Conn's syndrome) resulting from aldosterone-secreting adrenal adenoma
• To confirm primary aldosteronism (Sodium-depleted plasma renin test).

Patient preparation

• Explain to the patient that this test is used to determine the cause of hypertension.
• Tell the patient to discontinue or hold the use of diuretics, antihypertensives, vasodilators, oral contraceptives, and licorice as ordered by the doctor and to maintain a normal sodium diet (3 g/day) during this period.
• For the sodium-depleted plasma renin test, tell the patient that he'll receive furosemide (or, if he has angina or cerebrovascular insufficiency, chlorthiazide) and will follow a specific
  low-sodium diet for 3 days.
• The patient should not receive radioactive treatments for several days before the test.
• Tell him that the test requires a blood sample. Explain who wil perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes. Collect a morning
  sample, if possible.
• If a recumbent sample is ordered, instruct the patient to remain in bed at least 2 hours before the sample is obtained. (Posture influences renin secretion.) If an upright sample is ordered, instruct him to stand or sit upright for 2 hours before the test is performed.
• If renal vein catheterization is ordered, make sure the patient has signed an informed consent form. Tell him that the procedure will be done in the X-ray department and that he'll receive a local anesthetic.

Procedure and posttest care

Peripheral vein sample

• Perform a venipuncture, and collect the sample in a 7-ml lavender-top tube.
• Note on the laboratory slip if the patient was fasting and whether he was upright or supine during sample collection.
• If a hematoma develops at the peripheral venipuncture site, apply warm soaks.

Renal vein catheterization

• A catheter is advanced to the kidneys through the femoral vein under fluoroscopic control, and samples are obtained from both renal veins and the vena cava.
• After renal vein catheterization, apply pressure to the catheterization site for 10 to 20 minutes to prevent extravasation.
• Monitor vital signs, and check the catheterization site every 30 minutes for 2 hours and then every hour for 4 hours to ensure that the bleeding has stopped. Check distal pulse for signs of thrombus formation and arterial occlusion (cyanosis, loss of pulse, coolness of skin).

Both methods

• Tell the patient he may resume his usual diet.
• Resume administration of medications that were discontinued before the test.

Precautions

• Because renin is unstable, the sample must be drawn into a chilled syringe and collection tube, placed on ice, and sent to the laboratory immediately.
• Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant.

Reference values

Levels of plasma renin activity and aldosterone decrease with advancing age, as follows:

• Sodium-depleted, upright, peripheral vein: For ages 18 to 39, the range is from 2.9 to 24 ng/ml/hour; mean, 10.8 ng/ml/hour. For age 40 and over, range is from 2.9 to 10.8 ng/ml/hour; mean, 5.9 ng/ml/hour.
• Sodium-replete, upright, peripheral vein: For ages 18 to 39, range is from 0.6 to 4.3 ng/ml/hour; mean, 1.9 ng/ml/hour. For age 40 and over, the range is from 0.6 to 3 ng/ml/hour; the mean is 1 ng/ml/hour.

Abnormal findings

Elevated renin levels may occur in essential hypertension (uncommon), malignant and renovascular hypertension, cirrhosis, hypokalemia, hypovolemia due to hemorrhage, renin-producing renal tumors (Bartter's syndrome), and adrenal hypofunction (Addison's disease). High renin levels may also be found in chronic renal failure with parenchymal disease, end-stage renal disease, and transplant rejection.

Decreased renin levels may indicate hypervolemia due to a high-sodium diet, salt-retaining steroids, primary aldosteronism, Cushing's syndrome, licorice ingestion syndrome, or essential hypertension with low renin levels.

High serum and urine aldosterone levels with low plasma renin activity help identify primary aldosteronism. In the sodium-depleted renin test, low plasma renin confirms this and differentiates it from secondary aldosteronism (characterized by increased renin).

Interfering factors

• Failure to observe pretest restrictions
• Improper patient positioning during test
• Failure to use the proper anticoagulant in the collection tube, to completely fill it, or to adequately mix the sample and the anticoagulant (EDTA helps preserve angiotensin I; heparin does not)
• Failure to chill the collection tube, syringe, and sample or to send the sample to the laboratory immediately
• Salt intake, severe blood loss, ingestion of licorice, oral contraceptives, pregnancy, and therapy with diuretics, antihypertensives, or vasodilators(increase)
• Salt-retaining steroid therapy and antibiuretic therapy(decrease)
• Radioisotope use within several days before the test.